DSR2 Batch box in Nova Scotia

Last spring my wife and I started building a DSR2 batchbox rocket mass heater in our small strawbale cabin near Lunenburg, Nova Scotia, Canada.

A few weeks ago we fired it up for the first time and have been loving it so far! We wanted to thank all of the Rocket Scientists in this group for posting the information that allowed the build to happen, especially Peter for sharing your research and designs that we used for the heater.

Some additional info:
- 5" riser and flue
- DSR2 core made with firebrick using clay/sand mortar
- bell made from reclaimed clay brick with clay/sand mortar
- exterior 1" clay/sand plaster harvested from site + limewash
- bench/chair with cleanout and 5" flue joined existing 15' chimney with small bend
- firebox door from Salamander Stoves in the UK

So far, with overnight lows around -2C we have been having one 1-hour fire in the morning and staying nice and toasty with an even level of heat until the following morning.

We are in the process of building a larger house and hope to build a larger batchbox heater in a couple of years time. With that in mind, lessons learned so far:
1. We underestimated the importance of the size of the primary air intake. At first it was too small, leading to a couple of very smokey fires until it was enlarged to the correct dimensions.
2. Our flue is not insulated until it exits the roof and we now wish it was! Because of the lower temperature flue gases and uninsulated metal stovepipe we are getting quite a bit of condensation dripping out at the bottom.  We're planning to add some insulation at about the 7-12' level to hopefully reduce/eliminate the condensation.
3. Because of the small size of the cabin (375 square feet) the air pressure in the building does affect the draft in the flue. We have one extractor fan in our loft and when turned on it must reduce the pressure in the cabin enough to cause smoke to come out of the cleanout in the bench.
4. This one should have been obvious but it wasn't to us. It takes a few consecutive days of firing to get the entire mass from cold to 'operating' temperature. Ie. The bench doesn't warm up after the first couple fires and the "dragon" doesn't really come to life until day 3. At least that was our experience.
5. Even without lighting the heater we noticed a difference in temperature and humidity during the summer and fall due to the thermal mass. Fewer hot summer days and not as many cool autumn nights!

All in all we love our heater and are now trying to convince everyone we know to build one!

Happy to try to answer any questions or share construction details. Unfortunately we were too busy building to take photos of that process.

Thanks folks!
James & Kate

Good job Kate and James!

I understand that the two square holes are for the secondary air channel. Do you close it for the night?

Hi James,

Welcome to Permies.

Cristobal Cristo wrote:Good job Kate and James!

I understand that the two square holes are for the secondary air channel. Do you close it for the night?

Thanks Cristobal!

We have no option to close the primary/secondary air intakes overnight.  The secondary is simply a couple square tubes we had welded to spec for the floor channel. There is a damper in the exhaust flue that we close overnight.

Hi Guys, Thanks for sharing!
Do you monitor the outgoing exhaust gas temperature?
With only one fire a day, you may not be achieving or maintaining gas temps above 130°F (54 °C).
I highly recommend using a low-cost digital thermometer with a wire probe inserted into the outgoing chimney.
You don't need to use an insulated pipe indoors, as it won't lose enough heat inside the cabin.

With my Shorty core inside the house and only one fire a day, condensation is an issue.
I utilize my bypass to help even out the temps.
As soon as I switch from one fire a day, to one in the morning and another in the evening, my bypass can remain shut, and the outgoing gas temperatures range from 160°F to 200°F
Not a drop of black water anywhere.

I realize you are coastal rather than mountainous, and your temps will be more moderate.
Could you try two smaller fires a day and see if that helps?

With Shorty, we often have windows open, sometimes all night, even in northern Montana!

Here is the gauge I am using
https://www.amazon.com/Proster-Thermocouple-Thermometer-Dual-Channel-Thermocouples/dp/B071V7T6TZ/ref=sr_1_4?nsdOptOutParam=true&sr=8-4

Oh my James
Closing off your chimney is very dangerous!
Closing your secondary's is easy, and I think the DSR uses a similar door/ airframe as Shorty.
It should have a way to shut it off.
Check out this thread.
https://permies.com/t/361932/Holding-heat-overnight-importance-shutting

I hope it's a partially closing damper.
How is the primary air getting to the firebox?

Cristobal Cristo wrote:I hope it's a partially closing damper.
How is the primary air getting to the firebox?

Yes, partially closed, not to worry. The primary air is delivered through an intake below the door, just above the secondary intake, since the door didn't have its own intake.

Thanks Thomas, these are excellent points.

thomas rubino wrote:Do you monitor the outgoing exhaust gas temperature?
With only one fire a day, you may not be achieving or maintaining gas temps above 130°F (54 °C).
I highly recommend using a low-cost digital thermometer with a wire probe inserted into the outgoing chimney.
You don't need to use an insulated pipe indoors, as it won't lose enough heat inside the cabin.

Not yet other than by putting my hand on the stovepipe. It's barely warm to the touch so I would guess we are well below 54C.

thomas rubino wrote:

With my Shorty core inside the house and only one fire a day, condensation is an issue.
I utilize my bypass to help even out the temps.
As soon as I switch from one fire a day, to one in the morning and another in the evening, my bypass can remain shut, and the outgoing gas temperatures range from 160°F to 200°F
Not a drop of black water anywhere.

I realize you are coastal rather than mountainous, and your temps will be more moderate.
Could you try two smaller fires a day and see if that helps?

We'll definitely try this, especially with temperatures dropping soon. It would be fantastic if that solved the condensation issue. We didn't have the space to include a bypass with this layout but I will certainly be including one in our next build!

thomas rubino wrote:
Closing off your chimney is very dangerous!
Closing your secondary's is easy, and I think the DSR uses a similar door/ airframe as Shorty.
It should have a way to shut it off.

Sorry, didn't mean to alarm anyone. We can partially close it in an attempt to minimize heat loss. But after reading your other post it makes more sense to close the intakes. I'll try to come up with a way to do that, even if it's just covering with an insulating material.

Thanks!